The growth of large essentially single crystal ingots of various inorganic salts for use as optical bodies has been the focus of much attention over the past several decades. Among the salts especially suited for use as lenses over a wide range of wavelengths of radiation are the alkaline-earth metal halides. For many applications including, in particular, lenses used at wavelengths of 157 and 193 nm in photolithography equipment, a highly transparent, strain free and radiation hard single crystal, particularly of calcium or barium fluoride, is required. Another application that this technique is particularly suited for are scintillation crystals, such as Nal(TI) or Csl(TI), where oxygen impurities may act as sites where electrons are trapped.
Radiation hardness of a crystal is dependent on both the intrinsic properties of the crystal and the type and quantity of impurities in the crystal. Metallic impurities and oxygen within an alkaline- or alkali-earth halide crystal are known to decrease the radiation hardness of the crystal.
Radiation hardness may be defined as damage in a crystal resulting from exposure to energy that reduces the transmission of the crystal. It is also necessary to have a stoichiometric crystal as close to possible to theoretical to ensure good radiation hardness. In the case of alkaline-earth fluorides, this means two fluorine atoms for each alkaline-earth atom. In the case of alkali-earth fluorides, this means one fluorine atom for each alkali-earth atom. A desired stoichiometry is one that does not give unwanted absorbances at any wavelength, other than intrinsic absorbances.
In the present art, metals are removed by precipitation methods before converting compounds to the fluoride. Precipitation techniques are expensive and time consuming due to yield losses and manpower.
Oxygen is removed in alkaline- and alkali-earth halides by outgassing (heating under vacuum), which is a time consuming process. Some degassing occurs when a calcium/barium fluoride charge in a crucible is heated under vacuum and the absorbed species leaves by desorption. Although this heating under vacuum removes many of the unwanted contaminants, further improvements to the degassing are achieved when scavenging agents are added. In the present art, a scavenger is used to react with oxygen, which might be present in the raw material and crucible, to form a volatile oxygen compound that is exhausted from the crucible. Typical scavenging agents for alkaline- or alkali-earth fluorides include metal fluorides, hydrogen fluoride gas, polytetrafluoroethylene and carbon tetrafluoride. Perhaps the most widely used scavenging agent is lead fluoride.
Lead fluoride is typically added to the alkaline- or alkali-earth metal fluoride charge in quantities from 1 to 3 weight percent. The lead fluoride is added as a powder and is mechanically mixed with the alkaline- or alkali-earth metal fluorides such that it is uniformly dispersed to the extent possible. At high temperature, typically greater than 600° C., the lead fluoride begins to substantially volatize. Any oxygen, water, sulfur, sulfate species, as examples, can react with the lead fluorides and be exhausted as volatile lead compounds. This reduces the impurities in the charge and makes for products with improved characteristics of transparency, strain and radiation hardness.
Many of the contaminants in the alkaline- or alkali-earth metal fluorides may not be on the surface or may not reach the surface for discharge before the charge becomes molten and the crystal is grown. Consequently, impurities can still be trapped in the crystal, thereby degrading the final product.
U.S. Pat. No. 3,935,302 disclosed an attempt to provide single crystal refractory metal halides and rare earth halides for use as laser windows having excellent mechanical, thermal and optical properties. As described in this patent, laser windows were prepared from halide crystals purified by congruent growth from a melt using a reactive atmospheric processing method.